Field of the Invention
The present invention relates to an image sensor unit, a paper sheet distinguishing apparatus and an image reading apparatus. In particular, it relates to an image sensor unit that has a light source that irradiates an object to be illuminated with light, and a paper sheet distinguishing apparatus and an image reading apparatus that are provided with the image sensor unit.
Description of the Related Art
A paper sheet distinguishing apparatus or image reading apparatus incorporates an image sensor unit that reads an illuminated object, such as a bill or an original. The image sensor unit may include a light source that irradiates an object to be illuminated with light and an image sensor that detects light from the illuminated object. The light source deteriorates in irradiation intensity due to aging or the like. As the irradiation intensity of the light emitted from the light source decreases, the brightness of the read image decreases. To avoid this, the irradiation intensity of the light emitted from the light source is corrected to compensate for the decrease of the irradiation intensity of the light due to aging.
According to prior art, as disclosed in Patent Document 1, a white reference is used for such correction. More specifically, the light source emits light toward the white reference, the image sensor detects the light reflected from the white reference, and the irradiation intensity of the light of the light source is corrected based on the intensity of the detected reflected light. However, such an arrangement requires the white reference. In addition, since the correction is based on the light reflected from the white reference, the result of the correction is affected by optical characteristics of the white reference. For example, if the white reference has low reflectance, the intensity of the detected reflected light is also low, and therefore the light quantity required for the correction can be unable to be achieved.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-146285